Picture Mars, and let your mind's eye zoom in from orbit, through the whisper-thin atmosphere of carbon-dioxide, to the alluvial fan of Jezero Crater, where Perseverance awaits instructions.
Because Mars lacks a magnetosphere, it doesn't have protective Van Allen belts to shield the surface from the high-energy charged particles that comprise the solar wind. Lacking that protection and the protection of a thick, nitrogen rich atmosphere, bacteria on the surface of Mars would be hard-pressed to survive the constant radioactive bombardment from the sun and the rest of the cosmos.
So what about beneath the surface? If Perseverance discovers subsurface water (and that water occurs at a depth sufficient to escape the worst of the radioactive bombardment), could there have been life? Could there still be life? The challenge, of course, is this: what would those hypothetical bacteria or archaea use as an energy source?
Let your mind's eye zoom back to earth, but don't stop at the surface... we're heading to the bottom of the ocean.
At the bottom of the ocean, radiolysis (the disassociation of molecules such as water into their component atoms) is driven by relatively sparse but naturally occurring radionuclides. ("Radionuclides" are radioactively unstable isotopes such as Uranium-238, Radium-226, Thorium-228, or Lead-210). The result of this radiolysis of water is free molecular hydrogen (and oxygen, of course), which is of interest to us because there are relatively hardy bacteria that oxidize hydrogen as their energy source. (i.e., they eat H2, molecular hydrogen) Radiolytic hydrogen has been identified as the primary food source for microorganisms in continental aquifers kilometers below Earth’s surface, well outside the range of any possible photosynthesis.
The good news for the bacteria and archaea buried deep in the sediment of the ocean floor is that there are generally not enough radioluclides decaying to kill them via radiation poisoning. The bad news is that this also means there isn't enough molecular hydrogen being produced to use as an abundant food source.
Or is there?
Dr. Justine Sauvage (et al.) published a paper in Nature Communications yesterday examining the radiolysis of water under various conditions (pure water, ocean water, and ocean water mixed with a variety of marine sediment slurries) and measured the production of molecular hydrogen under each set of conditions. I'll let Dr. Sauvage take it from here:
"The extent to which most subsurface ecosystems rely on radiolytic products has been poorly constrained, due to incomplete understanding of radiolytic chemical yields in natural environments. Here we show that all common marine sediment types catalyse radiolytic H2 production, amplifying yields by up to 27X relative to pure water. In electron equivalents, the global rate of radiolytic H2 production in marine sediment appears to be 1-2% of the global organic flux to the seafloor. However, most organic matter is consumed at or near the seafloor, whereas radiolytic H2 is produced at all sediment depths. Comparison of radiolytic H2 consumption rates to organic oxidation rates suggests that water radiolysis is the principal source of biologically accessible energy for microbial communities in marine sediment older than a few million years. Where water permeates similarly catalytic material on other worlds, life may also be sustained by water radiolysis."
Because siliceous and calcareous oozes are composed largely of the silica and calcium carbonate remains of microbial organisms, finding them on Mars might be a long-shot. But lithogeneous and abyssal clay substrates, which are entirely abiotic? Finding their analogues on Mars is well within reasonable possibility.
No one who has seen photographs of the Jezero alluvial fan can look at it and fail to intuit that it was caused by flowing water.
Percy, we sent you a very long way to another world, and you've been a good boy. Is there water under that soil? Did something live in it? Does something still?
Percy, fetch!
(Back on earth, kudos to Dr. Sauvage for another great piece of research.)
Here is a link to a list of her other publications which are largely on the topic of biogeochemistry for all those interested or inclined!